The Great Space Coaster

Astronomers measure the Universe's deceleration before dark energy took over

For the past five billion years, the expansion of the Universe has been
speeding up, powered by the mysterious repulsive force known as "dark energy."
But thanks to a new technique for measuring the three-dimensional structure
of the distant Universe, astronomers from the Sloan Digital Sky Survey
(SDSS-III) have made the first measurement of the cosmic expansion rate
just three billion years after the Big Bang.

"If we think of the Universe as a roller coaster, then today we are rushing
downhill, gaining speed as we go," says Nicolás Busca of the Laboratoire
Astroparticule et Cosmologie of the French Centre National de la Recherche
Scientifique (CNRS), one of the lead authors of the study. "Our new measurement
tells us about the time when the Universe was climbing the hill — still
being slowed by gravity."

SDSS-III was able to measure the expansion rate of the distant Universe by using quasars to probe the distribution of hydrogen along the line of sight from the each quasar (red dots in the upper left) to the Sloan Foundation 2.5-m Telescope on Earth (abstractly represented as the plane in the lower right).+ MORE +

Light rays from distant quasars (dots at left)
are partially absorbed as they pass through clouds of intergalactic
hydrogen gas (center). When the light arrives at the spectrograph of
the Sloan Foundation 2.5-Meter Telescope (square at right), some
has been absorbed, leaving behind a record in the form of a "forest"
of small absorption lines in the observed spectrum.

These lines can be interpreted to make a map
of the gas along the line of sight between us and the quasar. By examining
light from thousands of quasars all over the sky, astronomers can make
a detailed three-dimensional map of the distant universe.

In this illustration, the dots at the far left
are quasars, and the thin lines show light rays that left those quasars
more than 10 billion years ago. Yellow dots are quasars that had been
measured by prior projects of the Sloan Digital Sky Survey. By measuring
the spectra from ten times as many quasars in this range (red dots),
BOSS can reveal the large-scale structure of the early universe in
much greater detail.- LESS -

The results were presented in a paper submitted to the journal Astronomy
& Astrophysics; the paper was also posted
today on the arXiv.org preprint site. The new measurement is based on data from
the Baryon Oscillation Spectroscopic Survey (BOSS),
one of the four surveys that make up SDSS-III. It utilizes a technique pioneered
by the SDSS in 2005 called "baryon acoustic oscillations (BAO)." The BAO technique
uses small variations in matter left over from the early Universe as a "standard ruler"
to compare the size of the Universe at various points in its history.

But using that ruler comes with its own difficulties. "If we want to make
a measurement at early times, then we need to map structure that is very far
away," says Jim Rich of Centre de Saclay Institute of Research into the
Fundamental Laws of the Universe (IRFU), another member of the analysis team.
"If we used galaxies, it would be very hard, because galaxies that are far
away are also very faint. So we have to try something else."

The new measurement does not look at galaxies at all. Instead, it makes use
of the clustering of intergalactic hydrogen gas in the distant Universe. We can
see this gas because it absorbs some light from quasars lying behind. When we
measure the spectrum of a quasar, we see not only the light emitted by the quasar,
but also what happened to that light in its long journey to Earth. When we look
at a quasar's spectrum, we can see how the intervening gas absorbs some of the
quasar's light. Measuring this absorption — a phenomenon known as the
Lyman-alpha Forest — yields a detailed picture of the gas between us
and the quasar.

"It's a cool technique, because we're essentially measuring the shadows
cast by gas along a single line billions of light-years long," says Ane
Slosar of Brookhaven National Laboratory. "The tricky part is combining all
those one-dimensional maps into a three-dimensional map. It's like trying to
see a picture that's been painted on the quills of a porcupine."

Last year, Slosar and his colleagues used the first 10,000 quasars from
SDSS-III's Baryon Oscillation Spectroscopic Survey (BOSS) to make the first
large-scale map of the structure of the faraway "Lyman-alpha forest" gas.
As enormous as that map was, it was still not large enough to detect the
subtle variations of BAOs. But the new map is big enough — it
measures the Lyman-alpha forest using light from 50,000 quasars all over
the sky.

When SDSS-III researchers began to study this bigger map more than a
year ago, they didn't know whether or not this technique would work. "When we started,
we didn't want to bias ourselves into seeing what we wanted to see," says team
member Timothee Delubac of Centre de Saclay IRFU. "We only looked at scales where we
didn't expect to see BAOs. But as we moved in on the right scale, we had a very
exciting moment. The BAO peak was sitting there in our plot, right where it would
be if dark energy were a constant property of space itself."

A new measurement from SDSS-III BOSS reaches twice as far into the past as previous direct measurements of the expansion rate (illustrated by the red dot on the left-hand side). The expansion rate of the Universe was slowing down 10 billion years ago, and started to speed up due to the influence of dark energy 6 billion years ago.
+ MORE +

Until recently, three-dimensional maps by
BOSS and other surveys were able to measure the regular distribution
of galaxies back to only about five and a half billion years ago, a
time when the expansion of the Universe was already accelerating.

The numbers along the bottom of the graph
show the time in the Universe's past, in billions of years. The
vertical scale (y-axis) shows the expansion rate of the Universe;
higher means the Universe was expanding faster. These older
measurements appear as data points toward the right of the graph.

The new SDSS-III measurements, shown as the
big red point to the far left, have now probed the structure of the early
Universe at a time when expansion was still slowing down.- LESS -

Credit: Zosia Rostomian, LBNL; Nic Ross, BOSS Lyman-alpha team, LBNL

The team's new measurement of the BAO peak, combined with measurements of the same
peak at other points in the Universe's history, paints a picture of how the Universe
has evolved over its history. The picture that emerges is consistent with our current
understanding of the Universe — that dark energy is a constant part of space
throughout the cosmos. What is fascinating about the new result is that, for the
first time, we see how dark energy worked at a time before the Universe's current
acceleration started.

"Our goal for BOSS was to measure the expansion of the Universe. We planned to
make that measurement in two ways -- one a sure thing and one a risky new idea," says
University of Utah astrophysicist Kyle Dawson, the BOSS survey scientist who ensures
that good data is taken for each of the 1.8 million galaxies and quasars that BOSS
will observe during the 6 years of SDSS-III. "It's really exciting that, thanks to
the dedicated work of so many people, we know that both methods work. We have
shown that the Lyman-alpha forest can accurately measure the expansion of the
Universe when it was only one-fifth its current age."

The BOSS measurements show that the expansion of the Universe was slowing
down 11 billion years ago due to the mutual gravitational attraction of all
of the galaxies in the Universe — but that as the Universe expanded, the
constant repulsive force of dark energy began to dominate as matter was diluted
by the expansion of space. Thus, more than eighty years after Edwin Hubble and
Georges Lemaître first measured the expansion rate of the nearby Universe,
the SDSS-III has made the same measurement of the expansion rate of the Universe
11 billion years ago.

"No technique has ever been able to probe this ancient era before," says
BOSS principal investigator David Schlegel of the Lawrence Berkeley National
Laboratory. "Back then, the expansion of the Universe was slowing down; today,
it's speeding up. How dark energy caused the transition from deceleration to
acceleration is one of the most challenging questions in cosmology."

SDSS-III will continue to learn more about dark energy as it collects more than
a million and half galaxies and more than 160,000 quasars by the end of the survey.
By the time SDSS-III is complete, it will have helped transform the Lyman-alpha
forest technique from a risky idea into a standard method by which astronomers
explore the nature of the faraway Universe.

Nicolás Busca summarizes: "It looks like the roller coaster crested the hill
just about seven billion years ago, and we're still going."

About SDSS-III

Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the
Participating Institutions, the National Science Foundation, and the U.S.
Department of Energy Office of Science. The SDSS-III web site is
http://www.sdss3.org/.

SDSS-III is managed by the Astrophysical Research Consortium for the Participating
Institutions of the SDSS-III Collaboration including the University of Arizona,
the Brazilian Participation Group, Brookhaven National Laboratory, University of
Cambridge, Carnegie Mellon University, University of Florida, the French
Participation Group, the German Participation Group, Harvard University, the
Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA
Participation Group, Johns Hopkins University, Lawrence Berkeley National
Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for
Extraterrestrial Physics, New Mexico State University, New York University, Ohio
State University, Pennsylvania State University, University of Portsmouth,
Princeton University, the Spanish Participation Group, University of Tokyo,
University of Utah, Vanderbilt University, University of Virginia, University of
Washington, and Yale University.